This invention relates to a knocking suppression apparatus for an internal combustion engine. More particularly, it relates to a knocking suppression apparatus which can detect knocking using only a single knock sensor.
A knocking suppression apparatus is a device which can stop or prevent knocking in an internal combustion engine. Knocking can be suppressed by adjusting the manifold pressure or the fuel supply, or most commonly, by adjusting the ignition timing of an engine. A typical knocking suppression apparatus includes a knock sensor in the form of an acceleration sensor. The knock sensor detects engine accelerations due to knocking and generates a corresponding electrical output signal. A signal processing circuit identifies signals due to knocking in the output signal of the knock sensor. When knocking is detected, the ignition coil of the engine is controlled so as to delay the ignition timing to the point where knocking no longer takes place.
In a conventional knocking suppression apparatus, a plurality of knock sensors are mounted directly on the cylinder block of an engine. FIG. 1 is a plan view of the cylinder block 1 of a conventional V-6 engine which is equipped with a knocking suppression apparatus. Reference numerals 11-16 indicate cylinder #1 through cylinder #6, respectively, of the engine. First through fourth knock sensors 17-20 in the form of acceleration sensors are mounted directly on the cylinder block 1 at four mounting locations 17a-20a, respectively, on the inner surfaces of the cylinder block 1 adjoining the two banks of cylinders. Each mounting location 17a-20a lies roughly midway between two adjacent cylinders of the engine.
FIG. 2(a) through 2(d) show the signal-to-noise ratio (S/N) of each knock sensor with respect to each cylinder of the engine. The signal-to noise ratio is the ratio of the level of the output of a knock sensor when knocking is taking place in one of the cylinders to the level of the output when there is no knocking. A signal-to-noise ratio of above a certain level is necessary to accurately detect knocking, and the higher the ratio, the better. The "near bank" is the bank of cylinders on which the knock sensor is mounted, while the "far bank" is the opposite bank of cylinders. Each knock sensor has a good S/N with respect to knocking by cylinders in the near bank, but it has a poor S/N with respect to knocking by cylinders in the far bank. For example, the first knock sensor 17 is sensitive to knocking by cylinders #1, #3, and #5, but it has poor sensitivity to knocking by the other three cylinders.
Since each knock sensor requires an individual signal processing circuit to identify knock signals in its output, the cost of a conventional knocking suppression apparatus is high. The cost is further increased by the time required to install the four knock sensors on an engine. It is therefore highly desirable to be able to reduce the number of knock sensors which are employed.
Ideally, a knock suppression apparatus would employ only a single knock sensor. With this object, numerous tests were performed to determined the best mounting location of a knock sensor on a V-6 engine. However, even under the best circumstances, a single knock sensor had an adequately high S/N for only 5 of the 6 cylinders. Accordingly, if a knock sensor is directly mounted on a cylinder block of a V-6 engine in the conventional manner, it is necessary to employ at least two knock sensors in order to obtain a satisfactory S/N for all six cylinders.
The same problem is experienced with knocking suppression apparatuses for in-line six-cylinder engines, which also require a plurality of knock sensors.